Contaminants in groundwater and soil are a significant environmental concern. Many industrial operations produce different types of contaminants that are introduced into water systems such as subsurface aquifers. The contaminants and the resulting contaminated groundwater form plumes that move through the aquifer. Such plumes may eventually enter areas of the aquifer from which water is normally drawn for agricultural or residential use, or may eventually crop out into other water systems such as streams and lakes.
The contaminants of concern encompass a wide variety of materials. These materials may be organic substances such as hydrocarbons (e.g., benzene, toluene, and related compounds, typically from petroleum) and industrial chemicals and solvents (e.g., trichloroethylene, tetrachloroethylene, pesticides, and similar compounds). The contaminants may also be inorganic elements and compounds such as metals (e.g., cadmium, chromium, and lead), radionuclides (e.g., uranium, plutonium, and others), and acids and bases. The contaminants may arise from any of a wide array of human technological and industrial activities such as chemical and nuclear production, processing, storage, transportation, and distribution.
One method of treating such contaminants involves first locating the plume of contaminated groundwater. Wells are drilled into the plume or at a location just ahead of the plume. The contaminated groundwater is extracted through the wells and passed to a processing operation. The processing operation may consist of reactor vessels or filters that operate either to remove the contaminants from the water or to mix the contaminated water with chemicals that neutralize, precipitate, or otherwise destroy the contaminants. The remediated water is then again injected into the ground.
This method of treating groundwater is energy and resource intensive. It can and does take months or years of constant pumping and treatment to successfully decontaminate a single contaminated plume. The energy expenditure for the pumping operations is very high, and the chemicals used to treat the contaminants must be continually replaced.
Another method of testing contaminated groundwater is to inject a treatment solution into the groundwater. The treatment solution is often an aqueous solution of a material or combination of materials. The material is injected into or ahead of the plume such that the plume mixes with the material. The material may react with the contaminants in a variety of reactions. One such reaction is precipitation, which effectively immobilizes the contaminant. Another type of reaction is one in which the material reacts with the contaminant to convert the contaminant to a harmless compound.
One promising method of decontamination of groundwater is the use of oxygen or oxidizing materials. Oxygen is useful for several different types of remediation of contaminants. For organic contaminants, an injection into the groundwater of oxygen or oxygen producing materials can enhance the growth of microbes native to the soil through which the contaminants flow. The microbes in turn utilize the organic contaminants as food sources, effectively converting them to harmless by-products. An example of this approach is found in U.S. Pat. No. 5,264,018 entitled “Use of Metallic Peroxides in Bioremediation” which issued Nov. 23, 1993 to Stephen Koenigsberg et al.
Oxygen can also enhance the precipitation, and hence immobilization, of certain metals, either by oxidizing metals that co-precipitate with the contaminant metals. Conversely, the presence of oxygen is also useful in enhancing the solubility in water of certain metals, such as chromium and uranium, enabling the extraction of the solubilized metals by filtration or other treatment.
Providing an oxygen rich environment is a contaminated plume, however, is problematic. Many materials other than the contaminants will take up oxygen and, because of its low solubility, oxygen will not adequately permeate a volume of soil or water when introduced from a single source or even a plurality of “point sources.” For the same reason, injected oxygen cannot spread far from the site of injection, meaning that the effective area of treatment is relatively small. Also, the formation of insoluble precipitates such as ferric hydroxide tend to clog the injection apparatus. These problems require the constant injection of oxygen, which is likewise energy and resource intensive; the placement of a large number of injection sites; and the constant unclogging of injection apparatus.
One recent attempt to provide an oxygenated environment for the treatment of contaminants involves the use of a metal peroxide. Magnesium peroxide, sold under the tradename ORC® (Regenesis, Inc., San Clemente, Calif.) has been placed in closely spaced wells or boreholes. This metal peroxide provides a slow, fairly constant release of oxygen in the presence of water. The oxygen stimulates the environmental clean up. The material is placed in permeable containers that are in turn placed in the wells. A significant drawback to this method is the limitation on placement of the material and exposure to the contaminants. To provide the needed oxygen enriched solution, many closely spaced wells must be drilled, requiring the expenditure of resources for the wells. Moreover, the geographic extent of the enriched oxygen is fairly limited, due to the fact that many materials that are not contaminants will utilize the available oxygen near its injection site at the borehole, preventing the peroxide or the released oxygen from significantly permeating the surrounding groundwater.
It has also been attempted to inject slurries of this product into subsurface areas in an attempt to treat contaminants. Typically, one or more boreholes are drilled and then filled with the slurry. The slurry material is immobile and, while releasing oxygen to the immediate environment, is subject to the drawbacks described above. This procedure requires closely spaced wells for injecting the slurry, and has not been found to adequately overcome the drawbacks mentioned above.
There is thus a continuing need for improvements in the remediation of contaminants in soil and groundwater.